Antennas constituted by a shell or casing, whereof one face called the mirror acts as a radiating or reflecting surface and a rigid structure producing the general shape of the mirror are widely used in various technologies using transmission by electromagnetic waves, such as for example in telecommunications, radar systems, etc.
With a view to facilitating the installation of such antennas and for reducing their cost, it is conventional practice to make the casing from a plastic material, which is optionally filled and generally has a honeycomb structure, the mirror surface being metallized. The mirror is fixed to the rigid support by screwed or adhered fastenings, in order to guarantee a rigid connection between the mirror and its support, thereby ensuring the overall rigidity of the latter. Thus, it is known that the performance characteristics of the antenna are largely determined by the surface of the mirror. With very small tolerances, it must be able to reproduce the theoretical surface defined by the sought characteristics of the antenna. On the basis of this objective, it is standard practice to fix the casing rigidly to its support.
In the design of radar antenna reflectors, it is conventional practice for the front structure forming the reflecting mirror to be constituted by a sandwich panel having a resin glass skin, whilst the rear support is constituted by a different structure, in the form of lattices or gratings of tubes, or shaped metal sections or sheet steel or aluminium boxes or panels.
These two structures are generally connected by rigid screwed or adhered fastenings.
The expansions of the front and rear structures are never strictly the same either because, as a result of insolation the temperature of the front, opaque, insulating face differs from that of the rear structure or because, even in the case of identical temperatures, the expansion coefficients of aluminium, steel and resin glass laminates are not the same.
Under the influence of these expansions, stresses occur at the connecting members between the front and rear structures, which lead to deformations of the mirror prejudicial to the satisfactory operation of the radar particularly when the sought radio performances require very strict mirror shape characteristics.